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As the generation mix evolves to a higher penetration of renewables and less traditional thermal generation, there are places in the power grid where stability becomes a challenge. It’s one that can be overcome by an unexpected technology—the synchronous condenser. Adding synchronous condensers can help with reactive power needs, increase short-circuit strength and thus system inertia, and assure better dynamic voltage recovery after severe system faults.
Synchronous condenser system up to 100 MVAr per machine
When a fault occurs on the grid the voltage is temporarily depressed, which requires an immediate and significant injection of vars to recover the voltage and prevent a collapse of the grid. In less severe fault situations or on a strong grid, an SVC is likely to be adequate. However, if there is a risk of more severe faults or the grid is weak, a synchronous condenser is the most secure solution. “The decision is based on a set of fault case assumptions that the utilities make for the grid and associated simulations,” adds Depoian. “But they know that if the voltage does not recover quickly, they violate grid standards and face the risk of a widespread blackout. Opting for a synchronous condenser may be the only solution or a much stronger solution.” While costs of synchronous condenser installations do vary, overall capital investment appears to be comparable to both SVC and STATCOM.
Inertia1 may also become an important concern in the not-too-distant future. The spinning mass in the grid—generally turbines and generators at the production end of the network—creates the system’s inertia. As conventional generation (nuclear, thermal) is retired, it is often replaced by wind or solar sources, which provide less inertia. As a result, frequency stabilization becomes more challenging. FACTS-based equipment cannot provide the necessary inertia. “The question of inertia and frequency stabilization is not yet critical,” Depoian points out, “though it is already a problem in some specific island-based grids. This is an instance where synchronous condensers with their spinning mass could help resolve the problem.”
1: Inertia is basically resistance to change in motion. Inertia of a power network helps limit frequency fluctuations when a power imbalance occurs.
Opting for synchronous condensers implies optimizing the specifications and design to fit the operator’s requirements. Parameters to be considered range from available substation footprint to long-term operating costs. The largest operating cost for synchronous condensers is electrical power losses, but certain choices can be made to reduce these:
As in many other technology domains, synchronous condensers have been constantly improved and upgraded over the years. Gone are the analog control systems and rheostats of the past that could not deliver adequate response to dynamic events. Today’s synchronous condenser comes complete with state-of-the-art digital controls and relays. Modern machines are available with either full static or brushless excitation. Full static excitation has faster response: however, many utilities prefer the lower initial cost and lower maintenance costs of brushless excitation systems. Brushless excitation uses a rotating exciter built into the shaft that is controlled by a digital automatic voltage regulator. The speed and precision of these devices allow the performance of the latest generation of synchronous condensers to exceed their SVC counterparts in some applications. Maintenance needs are also considerably reduced. Depoian comments: “They still fulfill the same function, but their technology, quality and reliability have progressed almost beyond recognition.”
Synchronous condenser system up to 200 MVAr based on Topair or Topack generator combined with a FKG generator circuit breaker
German TSO TenneT called on GE Grid Solutions to adapt and install a two-pole synchronous condenser. “We needed it for high-speed dynamic voltage support and short-circuit power in case of failure on our grid,” says TenneT System Technology Engineer Dr. Simon Konzelmann. It was installed at their Bergrheinfeld substation in Bavaria, where the nearby Grafenrheinfeld nuclear power plant was recently closed down.
Synchronous condenser installed with GCB and busduct
FKG generator circuit breaker
Topair generator (50WY23Z-124 type)
The solution is based on the “Topair” range generator. This large rotating machine is normally part of a power plant. “Here, for the first time, it is installed by a TSO at one of its substations,” says Dr. Gert Hentschel, Grid Solutions Technical Solutions Manager. “The challenge was to build a standardized, yet flexible system that fits within its substation environment.” An integrated solution was designed with the assembly of a set of proven products and modern digital controls. The protection and control of the synchronous condenser and its 400 kV substation bay had to be developed for the TSO, as did the interlocking systems of the switchgear bay and the generator circuit-breaker components. The solution was installed in December 2015 and has been in trial operation since. A number of adjustments have been made to the control and protection system and the excitation has been adapted. However, as Konzelmann stresses, “so far, we are satisfied that the synchronous condenser solution does what we expected of it.
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